The present invention relates to electrical shunting switch assemblies designed for use with electrolytic cells. The switch assembly acts as a parallel current carrying shunt path around the electrolytic cell when the switches of the assembly are in the closed, current carrying position. The electrolytic cell may then be serviced while other cells in a multi-cell system remain in operation. The switch assembly is thereafter actuated to open the switches and divert current back through the cell when the cell is to be connected back in the system. More particularly this electrical shunting switch assembly utilizes a plurality of vacuum switches which are electrically connected in parallel path relationship with a series connected predetermined resistance value in each parallel path, and including means for asynchronously operating the vacuum switches. The shunting switch assembly is adapted to be electrically connected in parallel across the terminals of the electrolytic cell. The vacuum switches of the present invention are more particularly adapted to be utilized with a diaphragm or membrane type electrolytic cell.
The term electrolytic cell applies to a variety of electrochemical devices ranging from electrolytic metal refining devices to more widely used chlor-alkali cells. These latter chlor-alkali electrolytic cells rely upon the passage of a DC electric current through an alkali metal halide solution to separate useful chemical constituents. The most widely used such chlor-alkali cells are mercury cells in which mercury is used as one of the electrodes of the device, and alkali metal hydroxide and halogen gas are produced. The use of vacuum electrical shunting switches with such mercury type cells is described in U.S. Pat. No. 4,075,448. The use of such vacuum type cell bypass or shunting switches with such mercury cells results in improved efficiency of operation of the cells, as well as reliable and simplified maintenance and operation of the cells. The layout of a mercury cell plant is such that it has been the practice to connect in place as a permanent connection, one or more of the vacuum type switches described in the aforementioned patent.
In a diaphragm type chlor-alkali electrolytic cell, one or more diaphragms which are permeable to the flow of electrolyte solution are utilized to separate the halogen gas and the alkali metal hydroxide. In a membrane type electrolytic cell one or more membranes or ion-exchange barriers or membranes, are utilized to effect separation of the alkali metal hydroxide and halogen gas. Such diaphragm and membrane type cells are generally more compact in their physical layout and require less frequent periodic maintenance. It has thus been the practice to utilize portable electrical cell shunting assemblies with such diaphragm type cells with conventional knife-edge or air-exposed breaker type electrical switches. In general, diaphragm and membrane type cells carry higher operating currents, and thus impose more severe current interrupting capability upon the electrical bypass or shunting switch assemblies.
The electrical shunting switch utilized with a diaphragm or membrane cell must be capable of carrying and interrupting the very high DC currents of the system, which currents range up to several hundred thousand amperes. The shunting switch must interrupt current in the shunt path when the cell is to be placed back in series operation with the other cells of a plant. A significant amount of energy must be dissipated in the vacuum switch during interruption of the high bypass current.
An earlier vacuum switch assembly which was capable of portable connection to such diaphragm or membrane type cells is described in U.S. Pat. No. 4,302,642 filed Aug. 24, 1977 entitled "Vacuum Switch Assembly". In this earlier shunting switch assembly, a plurality of vacuum switches are connected in parallel path relationship with individual parallel electrical bus conductors extending from each switch contact in electrical parallel isolated relationship from each side of the switch to the respective cell terminals. The resistance values of these electrical bus conductors and the physical relationships were such as to minimize self-inductance and mutual inductance effects so that the energy which must be dissipated in the last-to-open vacuum switch interrupter is minimized. In this earlier switch assembly a common operating drive mechanism was utilized to approximately simultaneously open the vacuum switch contacts during current interruption. It is known that it is physically impossible to exactly simultaneously open such a plurality of switches. The last-to-open switch contacts will thus carry the total current in the parallel path shunt assembly. It was the purpose of this earlier switch assembly to reduce the energy which must be dissipated in the last open switch.
It is a general objective in designing vacuum switch electrical shunting assemblies to minimize the arc current which the switch contacts are called upon to interrupt and dissipate. The vacuum switches are designed for long lived, reliable switch operation at a predetermined design level or rating.